During combustion, an internal combustion engine oxidizes gasoline and combines hydrogen (H2) and carbon (C) with air. Combustion creates chemical compounds such as carbon dioxide (CO2), water (H2O), carbon monoxide (CO), nitrogen oxides (NOx), unburned hydrocarbons (HC), sulfur oxides (SOx), and other compounds. During an initial startup period after a long soak, the engine is still “cold” and combustion of the gasoline is incomplete. A catalytic converter treats exhaust gases from the engine. During the startup period, the catalytic converter is also “cold” and does not operate optimally.
In one conventional approach, an engine controller commands a lean air/fuel (A/F) ratio thereby reducing the mass of liquid fuel to the engine. More air is available relative to the mass of liquid fuel to sufficiently oxidize the exhaust CO and HC. The resulting exhaust products then heat-up the catalytic converter. However, operating the cold engine at the lean condition can reduce engine stability, adversely impact vehicle drivability, and limits the catalyst warm-up rate.
In another conventional approach, the engine controller commands an excessively fuel-rich mixture which provides stable combustion, good vehicle drivability and excess CO and HC for generating exhaust heat. A secondary air injection system is added to inject air into the rich combustion exhaust products and provide an overall lean exhaust A/F ratio. The additional injected air oxidizes the excess CO and HC in the exhaust, and this releases heat for fast warm-up of the catalytic converter. The warmed catalytic converter then further oxidizes CO and HC in the oxidizing environment provided by the air injection system. Once the catalyst reaches operating temperature, the excess air from the air injection system must be eliminated to provide a stoichiometric exhaust A/F ratio for the catalyst to simultaneously reduce HC, CO and NOx emissions.
To ensure consistent CO and HC exhaust combustion, secondary air injection systems typically require much richer A/F ratios (20-30% more) than would be required for stable engine operation. However, injecting excess liquid fuel into a cold engine to generate a very fuel-rich exhaust mixture leads to excessive fuel wetting of the engine intake and cylinder surfaces. This results in high, uncontrolled HC emissions, oil contamination, spark ignition problems and increased fuel consumption.